## **Core Concept**
Bence Jones proteins are a type of abnormal protein found in the urine of patients with certain types of blood cancers, most notably **multiple myeloma**. These proteins are essentially **light chains** of immunoglobulins (antibodies) that are produced by malignant plasma cells. Their presence in urine is a diagnostic marker for multiple myeloma and other plasma cell dyscrasias.

## **Why the Correct Answer is Right**
The correct answer, **B. Light chains of immunoglobulins**, is right because Bence Jones proteins are indeed the light chains (either kappa or lambda) of immunoglobulins. In multiple myeloma, a single clone of plasma cells proliferates excessively, producing large amounts of a single type of immunoglobulin or its components, including light chains. When the production of light chains exceeds the capacity of the kidneys to reabsorb them, they spill into the urine, where they can be detected as Bence Jones proteins.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because Bence Jones proteins are not **heavy chains of immunoglobulins**. Heavy chains are larger components of immunoglobulins, and while they can also be produced in excess in some plasma cell dyscrasias, Bence Jones proteins specifically refer to light chains.
- **Option C:** This option is incorrect because Bence Jones proteins are not **complement proteins**. Complement proteins are part of the innate immune system and help clear pathogens from an organism, distinct from the immunoglobulins and their components.
- **Option D:** This option is incorrect because Bence Jones proteins are not **cell membrane proteins**. While plasma cells, like other cells, have cell membrane proteins, Bence Jones proteins are specifically related to the abnormal production of immunoglobulin light chains.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the detection of Bence Jones proteins in urine requires **urine protein electrophoresis** or **urine immunofixation electrophoresis**, as they may not be detected by routine urine dipstick tests. This is because the dipstick test primarily detects albumin, not light chains.

## **Correct Answer:** B. Light chains of immunoglobulins